Acknowledgments The translator would like to thank the staff of the editorial offices of the Einstein Papers in Boston for their cooperation, particularly Dr. Michel Janssen for his careful review of certain letters related to his expertise and for helpful comments made by Christopher J. Smeenk, University of Pittsburgh. Giuseppe Castagnetti, Berlin, provided the translation of the Italian letters by Levi-Civita. Particular thanks also to Alice Calaprice, Senior Editor at Princeton University Press, for her ongoing support and assistance in the preparation and production of this volume.